Ignition devices for pyrotechnic occupant protection devices in motor vehicles, for example airbags, belt tighteners, etc. have to have a high level of functional reliability. Ignition devices such as these are subjected to a so-called “Bruceton” test for quality assurance. In this case, a number of samples are taken from a batch of ignition units, for example, ignition pellets. A first voltage is then applied to a first ignition unit. If the ignition unit still does not ignite at this voltage, the voltage is increased until ignition takes place. After the ignition of the ignition unit, the voltage at which the first ignition unit ignited is somewhat reduced for a second ignition unit. If the second ignition unit does not ignite, its voltage is once again slightly increased until ignition takes place, etc.
A further test, frequently a 100% test, that is to a say a test which is carried out with every ignition unit that is manufactured, is the so-called “Thermal Transition Test (TTT)”. If there is a poor connection between the pyrotechnic device and an ignition wire of the ignition unit, it is in some circumstances possible for the ignition wire to melt without the pyrotechnic device being ignited. A poor connection may occur, for example, as a result of a bubble in the pyrotechnic material, or as a result of dirt on the ignition wire. In order to make it possible to assess the contact between the ignition wire and the pyrotechnic device, the ignition wire has a defined current applied to it, which is well below the triggering threshold. Depending on the connection to the pyrotechnic device, the ignition wire changes its resistance, with a small resistance change indicating a good connection, and a high resistance change indicating a poor connection.
Conventional ignition devices have connections via which the ignition unit is directly electrically accessible, for example a melting wire. In modern ignition devices, which are driven via an ignition bus and have their own integrated electronics units (smart igniter technology), the melting wire of the ignition unit is no longer directly electrically accessible. Instead of this, the ignition bus is connected to an electronics unit, which produces a well-defined and stabilized ignition voltage at a voltage output from a supply voltage which is provided on the ignition bus, and allows energy to be supplied to the ignition pellet only when an ignition signal is supplied via the ignition bus. This results in a high level of operational safety. However, the described tests are no longer possible, since the ignition element itself, such as a melting wire, a thin film element, a thick film element, a so-called silicon bridge wire etc., is not accessible for voltage to be applied directly.